The technique of washing your arse after having a shit and not having any other way to clean your arse. the double flush is done by flushing the toilet, cupping your hand and cleaning your arse with the flush water.

To double flush (verb). Used to describe the necessity to flush a toilet twice when a fecal explusion from the body is exceptional in terms of quantity (i.e., pure mass) and/or quality (i.e., lettuce).

Mother: "Boy were you ever in the bathroom a long time."
Son: "I think that was the biggest crap in my life. It was so big I had to double flush."